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        <h3 id="一-let关键字"><a href="#一-let关键字" class="headerlink" title="一.let关键字"></a>一.let关键字</h3><h4 id="特性"><a href="#特性" class="headerlink" title="特性"></a>特性</h4><h5 id="1-let关键字的闭包特性"><a href="#1-let关键字的闭包特性" class="headerlink" title="(1)let关键字的闭包特性"></a>(1)let关键字的闭包特性</h5><blockquote>
<p>列子一</p>
</blockquote>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">for</span>(<span class="keyword">let</span> i = <span class="number">0</span>;i&lt;<span class="number">10</span>;i++)&#123;</span><br><span class="line">    global.setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="string">"使用let"</span>+i);</span><br><span class="line">    &#125;,<span class="number">10</span>*i);</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">// console.log(i);//ReferenceError这里报错</span></span><br><span class="line"><span class="keyword">for</span>(<span class="keyword">var</span> a =<span class="number">0</span>;a&lt;<span class="number">10</span>;a++)&#123;</span><br><span class="line">    global.setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="string">"使用var"</span>+a);</span><br><span class="line">    &#125;,<span class="number">0</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(<span class="string">"输出的结果是var的i:"</span>+a);</span><br></pre></td></tr></table></figure>
<ul>
<li>输出:</li>
</ul>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line">输出的结果是var的i:10</span><br><span class="line">使用let0</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用var10</span><br><span class="line">使用let1</span><br><span class="line">使用let2</span><br><span class="line">使用let3</span><br><span class="line">使用let4</span><br><span class="line">使用let5</span><br><span class="line">使用let6</span><br><span class="line">使用let7</span><br><span class="line">使用let8</span><br><span class="line">使用let9</span><br></pre></td></tr></table></figure>
<p>这个例子可以体现给出let具有的以下几个特性</p>
<ol>
<li><p>自闭包性:从使用var和使用let输出的差异性可以看出来,for循环(或者while循环)使用let的时候值就是自己循环时候的值而不是最终值</p>
</li>
<li><p>块级作用于独占性:let只有在自己的作用于才有效,上面的ReferenceError报错可以体现出来,注意:for循环中设置变量的时候也是一个单独的作用域(有坑点见下面的例子)</p>
</li>
</ol>
<blockquote>
<p>例子二</p>
</blockquote>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//for循环1</span></span><br><span class="line"><span class="keyword">for</span>(<span class="keyword">let</span> i = <span class="number">0</span>;i&lt;<span class="number">10</span>;i++)&#123;</span><br><span class="line">    global.setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="string">"第一个for循环:"</span>+i);</span><br><span class="line">    &#125;,<span class="number">100</span>);</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">//for循环2</span></span><br><span class="line"><span class="keyword">for</span>(<span class="keyword">let</span> i = <span class="number">0</span>;i&lt;<span class="number">10</span>;i++)&#123;</span><br><span class="line">    <span class="keyword">let</span> i=<span class="number">1</span>;</span><br><span class="line">    global.setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">        <span class="built_in">console</span>.log(<span class="string">"第二个for循环:"</span>+i);</span><br><span class="line">    &#125;,<span class="number">200</span>);</span><br><span class="line">    i++;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<ul>
<li>输出</li>
</ul>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">0</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">1</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">3</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">4</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">5</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">6</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">7</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">8</span></span><br><span class="line">第一个<span class="keyword">for</span>循环:<span class="number">9</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br><span class="line">第二个<span class="keyword">for</span>循环:<span class="number">2</span></span><br></pre></td></tr></table></figure>
<ul>
<li>第二个for循环只输出 2 是因为 let虽然提供了闭包的特性但是没有在本质上改变js的特性,是有循环2在到达时间的时候候读取的是自己块级作用域中的let元素的值</li>
</ul>
<blockquote>
<p>例子三</p>
</blockquote>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//变量提升使用 var 的情况</span></span><br><span class="line"><span class="built_in">console</span>.log(foo); <span class="comment">// 输出undefined</span></span><br><span class="line"><span class="keyword">var</span> foo = <span class="number">2</span>;</span><br><span class="line"><span class="comment">//变量提升使用 let 的情况</span></span><br><span class="line"><span class="built_in">console</span>.log(bar); <span class="comment">// 报错ReferenceError</span></span><br><span class="line"><span class="keyword">let</span> bar = <span class="number">2</span>;</span><br><span class="line"><span class="keyword">var</span> ddd=<span class="number">3234</span>;</span><br><span class="line"><span class="keyword">let</span> ddd=<span class="number">123</span>;<span class="comment">//SyntaxError 只能有一个参数let和var 其实在一定程度上是相同的</span></span><br><span class="line"><span class="built_in">console</span>.log(ddd);</span><br></pre></td></tr></table></figure>
<ul>
<li>这个例子可以体现给出let的以下几个特性:</li>
</ul>
<ol>
<li><p>let和var不同:let使用必须先进行声明否则会报错,var会输出underfind</p>
</li>
<li><p>let和var不同:let不允许和var或者let变量在相同的作用域下同名</p>
</li>
</ol>
<blockquote>
<p>例子四<strong>暂时性死区</strong></p>
</blockquote>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> i=<span class="number">123</span>;</span><br><span class="line">&#123;</span><br><span class="line">    <span class="keyword">let</span> i=<span class="number">222</span>;</span><br><span class="line">    <span class="built_in">console</span>.log(i);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<ul>
<li>输出</li>
</ul>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="number">222</span></span><br></pre></td></tr></table></figure>
<ul>
<li>这个例子可以体现给出let的以下特性:let属性区域性独立,外部不会影响到内部,大多数变成语言都这个特性</li>
</ul>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (<span class="literal">true</span>) &#123;</span><br><span class="line">    <span class="comment">// TDZ开始</span></span><br><span class="line">    tmp = <span class="string">'abc'</span>; <span class="comment">// ReferenceError</span></span><br><span class="line">    <span class="built_in">console</span>.log(tmp); <span class="comment">// ReferenceError</span></span><br><span class="line">    <span class="keyword">let</span> tmp; <span class="comment">// TDZ结束</span></span><br><span class="line">    <span class="built_in">console</span>.log(tmp); <span class="comment">// undefined</span></span><br><span class="line">    tmp = <span class="number">123</span>;</span><br><span class="line">    <span class="built_in">console</span>.log(tmp); <span class="comment">// 123</span></span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">bar</span>(<span class="params">x = y, y = <span class="number">2</span></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> [x, y];</span><br><span class="line">&#125;</span><br><span class="line">bar(); <span class="comment">// 报错</span></span><br></pre></td></tr></table></figure>
<p>这个例子可以体现给出let的以下特性:TDZ</p>
<ol>
<li><p>在变量声明前都会不能使用</p>
</li>
<li><p>只要出现生命前使用了变量都会出现TDZ错误</p>
</li>
</ol>
<blockquote>
<p>坑点记录</p>
</blockquote>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> tmp = <span class="keyword">new</span> <span class="built_in">Date</span>();</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="built_in">console</span>.log(tmp);</span><br><span class="line">    <span class="keyword">if</span> (<span class="literal">false</span>) &#123;</span><br><span class="line">        <span class="keyword">var</span> tmp = <span class="string">'hello world'</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line">f(); <span class="comment">// undefined 因为变量提升的问题所以var是可以判断块级作用域的</span></span><br><span class="line"><span class="keyword">var</span> s = <span class="string">'hello'</span>;</span><br><span class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i = <span class="number">0</span>; i &lt; s.length; i++) &#123;</span><br><span class="line">  <span class="built_in">console</span>.log(s[i]);</span><br><span class="line">&#125;</span><br><span class="line"><span class="built_in">console</span>.log(i); <span class="comment">// 5 使用var会将for循环中的属性暴露在全剧中</span></span><br></pre></td></tr></table></figure>
<h3 id="二-const关键字"><a href="#二-const关键字" class="headerlink" title="二.const关键字"></a>二.const关键字</h3><p>这个关键字的左右就是常量,但是他是锁定变量的地址的,其余的和let相同</p>
<ul>
<li>注意:因为是锁定地址的所以需要进行对象的锁定是不安全的(对象中也有指向地址的东东)</li>
</ul>
<p>如果要实现锁对象那么就使用</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> foo = <span class="built_in">Object</span>.freeze(&#123;&#125;);</span><br><span class="line"><span class="comment">// 常规模式时，下面一行不起作用；</span></span><br><span class="line"><span class="comment">// 严格模式时，该行会报错</span></span><br><span class="line">foo.prop = <span class="number">123</span>;</span><br></pre></td></tr></table></figure>

      
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              <div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-3"><a class="nav-link" href="#一-let关键字"><span class="nav-number">1.</span> <span class="nav-text">一.let关键字</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#特性"><span class="nav-number">1.1.</span> <span class="nav-text">特性</span></a><ol class="nav-child"><li class="nav-item nav-level-5"><a class="nav-link" href="#1-let关键字的闭包特性"><span class="nav-number">1.1.1.</span> <span class="nav-text">(1)let关键字的闭包特性</span></a></li></ol></li></ol></li><li class="nav-item nav-level-3"><a class="nav-link" href="#二-const关键字"><span class="nav-number">2.</span> <span class="nav-text">二.const关键字</span></a></li></ol></div>
            

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